Optical fibers which are strong and have very few intrinsic defects are suitable for use for light transmission. However, optical fibers are very easily flawed by exposure to environmental conditions, including dust and moisture, and even small flaws can significantly reduce the strength of a fiber, rendering the fiber brittle and easily broken by a weak external force. Accordingly, optical fibers have conventionally been provided with at least one resin coating, preferably immediately after preparation of the optical fibers, to protect the fibers from exposure to conditions which would cause detrimental defects.
At least two resin coatings are often provided on an optical fiber, namely a primary or buffer inner coating and a secondary outer coating. Generally, the primary inner coating is applied directly to the glass fiber and, when cured, forms a soft, rubbery material which serves to cushion and protect the fiber by relieving stresses which are created when the fiber is bent, cabled or spooled. Such stresses might otherwise induce microbending of the fibers and cause undesirable attenuation of light traveling through the fibers. The secondary outer coating is usually applied directly over the primary coating and, when cured, forms a hard, tough outer layer which protects the glass fiber from abrasion, moisture and other effects which can damage the glass fiber. The Shustack U.S. Pat. Nos. 5,146,531 and 5,352,712 disclose optical fibers containing both primary and secondary coatings and optical fibers containing only the secondary outer coatings.
In order to provide coated optical fibers which are suitable for use under various conditions, the coatings which are applied to the glass fibers must exhibit certain combinations of desirable physical properties. For example, the primary coating must maintain adequate adhesion to the glass fiber during thermal and hydrolytic aging, yet be strippable for splicing purposes. The modulus of the primary coating must be low to cushion and protect the fiber, particularly through a wide temperature range to which the coated fiber may be exposed during its lifetime. The primary coating should also have a relatively high refractive index and high resistance to moisture.
The secondary coating should provide a hard protective layer which protects the glass fiber during processing and use. The secondary coating should therefore have a high glass transition temperature, i.e., at least about 50.degree. C., and a high modulus, i.e., at least about 40,000 psi and more preferably at least about 70,000 psi. The secondary coating, like the primary coating, should also exhibit high moisture resistance, a high refractive index, and good optical clarity.
Various primary and secondary coatings for optical fibers are known in the art. While the inner primary coating may be omitted, virtually all optical fibers require the hard protective secondary outer coating. As the optical fibers are formed by drawing, they are typically coated with both primary and secondary coatings or with only a secondary coating, and then immediately subjected to radiation, typically ultraviolet (UV) radiation, to cure the compositions. The coated fibers are then arranged on a spool for storage, shipment and use. From time to time, the spooled fibers are subjected to temperature cycling conditions, i.e., from hot to cold, or vice versa, which can result in attenuation of light traveling through the fiber and therefore a loss of signal. Snagging of adjacent fibers has also been observed during the expansion which accompanies such temperature cycling. This snagging between adjacent fibers also results in attenuation of light traveling through the fibers. It is believed that the attenuation losses which result from temperature cycling of the spooled fiber can be a result of the surface characteristics of the optical fiber's secondary coating, particularly that the coefficient of friction of the secondary coating is not sufficiently low to allow the fibers to slide easily relative to one another.
Various conventional additives are available for reducing the coefficient of friction of cured resin compositions. Typically, however, the addition of conventional coefficient of friction-reducing agents to secondary coating compositions for optical fibers significantly and detrimentally effects one or more properties of the compositions, such as optical clarity, modulus or the like, thereby rendering the compositions unsuitable for use as a secondary coating for optical fibers. Particularly, conventional coefficient of friction-reducing agents such as silicone acrylate materials exhibit a degree of incompatibility with the aliphatic urethane oligomers which are often used as a base resin for secondary coating compositions. As a result of this incompatibility between the coefficient of friction-reducing agent and the base resin, the compositions exhibit varying degrees of haziness, opaqueness and/or coloration, surface blooming, reduced modulus and the like. Accordingly, it has been difficult to provide optical fibers with secondary coatings which exhibit a desirable combination of physical properties and a reduced coefficient of friction which allows spooled fiber to withstand temperature cycling without significant attenuation or loss of signal.